1. Field of the Invention
The present invention relates to a competition avoidance control method for a data transmission-reception system in which a base station and a plurality of terminals are connected to prevent competition for priority over data reception among the terminals during the period from when the respective terminals receive an indication signal simultaneously until they receive an indication signal, a data transmission-reception system, and a terminal for a data transmission-reception system.
2. Description of the Prior Art
FIG. 1 is a sequence diagram to explain a conventional method for controlling a wireless LAN as a data transmission-reception system.
FIG. 1 shows an example of data reception sequence in stations or terminals (hereinafter referred to as STA) in the power save (hereinafter referred to as PS) mode of the infrastructure of a wireless LAN in which an access point (hereinafter referred to as AP) as a base station and a plurality of terminals are wirelessly connected.
In FIG. 1, the shaded portions of each of the STAs 1 and 2 show electricity consumption.
The AP cyclically sends an annunciation signal called Beacon to all the STAs. The beacon contains information about the presence or absence of data to each STA.
In FIG. 1, each STA consumes electricity to receive the beacons. The STA 1 learns that there is data to the STA 1 by the second beacon and sends a packet called PS-POLL to the AP. Having received the PS-POLL, the AP recognizes that the STA 1 is ready for data reception, thus sending data to the STA 1. After receiving this data, the STA 1 sends an acknowledgement packet ACK to the AP. The STA 1 consumes electricity in sending the PS-POLL, receiving the data, and sending the ACK.
The operation of the conventional data transmission-reception system for a plurality of STAs will be described referring to FIG. 2.
FIG. 2 is a sequence diagram to explain the conventional method for controlling a wireless LAN for a plurality of terminals.
When both the STA 1 and STA 2 have data to be received, each STA tries to send its PS-POLL to the AP after receiving a beacon. The STA which failed to acquire a priority to send the PS-POLL (STA 2 in FIG. 2) is in the wait state (standby mode) until the STA 1 finishes receiving the data. Because it is unclear when the STA 1 finishes receiving the data, the STA 2 always monitors electric wave condition and also is ready for transmission, and therefore consumes electricity all the while.
The case that there is data for one STA is shown in FIG. 1.
On receipt of a beacon indicating that the AP has data for the STA 1, the STAL sends a PS-POLL to the AP just after finishing beacon receiving process. The AP learns by receiving the PS-POLL that the STA 1 is ready for reception, and sends the data to the STA 1 in response to the PS-POLL.
The case that there are data for a plurality of STAs is shown in FIG. 2. On receipt of beacons indicating that the AP has data for both the STA 1 and STA 2, the respective STAs try to send a PS-POLL just after having received a beacon as in the case of FIG. 1.
However, to avoid radio collision, these two STAs are given priority on a first-come basis (competition).
In the following, a description will be given of the competition.
Normally, for power saving, the STAs switch to receiving mode at beacon intervals, and otherwise turn off the transmission-reception circuit to enter the mode consuming as little electricity as possible. In other words, when the AP simply sends data to the STA, the STA cannot receive the data because the reception circuit is off. Therefore, the AP sends a beacon to the STA and waits for a response, PS-POLL. When the AP has data to send to some STAs (the same or different data may be sent to the respective STAs), the AP sends the data to the STAs in order of receipt of their PS-POLLs. Incidentally, the STA arbitrarily sends data to the AP. The AP is always ready to send and receive data, and therefore, is capable of sending data immediately if the radio wave is available.
When sending a PS-POLL, the STA monitors the radio wave condition. If the radio wave is available, the STA sends the PS-POLL. Thus, the respective STAs send a PS-POLL on a first-come basis.
In FIG. 2, the STA 1 won the priority competition. Having failed to acquire the priority, the STA 2 waits for the STA 1 to finish data receiving process while monitoring the radio condition because the time which it takes for the STA 1 to complete the process is unclear. The STA 2 sends a PS-POLL to the AP immediately after the STA 1 has finished data receiving process to receive data. The STAs waste electricity after failing to acquire the priority until sending the PS-POLL.
Consequently, for example, the Japanese Patent Application Laid-Open No. HEI 8-195754 has proposed a radio communication device intended to reduce electricity consumption.
However, according to the conventional technique mentioned above, because the standby time of a mobile station is set longer every time the beacon signal is received, there is a high possibility that an interrupt from another mobile station occurs during the standby time. On the other hand, because standby time is set short while the beacon signal is not received, there is a possibility that the mobile station wastefully consumes electricity. In addition, because the timer operation is stopped when the standby time is shorter than a designated length, there is also a possibility that the mobile station wastefully consumes electricity.